drosophila disease ?

We have been breeding Drosophila for years on standard medium (corn meal,
agar, dry yeast and nipagine). But now adults die and fist instar larva
do not enter the medium but stay on the surface and climb on the walls of
the vials. No bacterial or fungic contamination, nor mite were detected.
Can you help us please? we are loosing our strains!
Thank you in advance.